After some flirtatious overtures, the mob boss's daughter will get into her convertible with Niko, and the two will go for a test drive. It's absolutely integral that you don't deviate from the path laid out on your HUD. If you do, the girl will become suspicious of what you're doing, but if you stay on the path designated, you won't alert her that something's up until the right time. So, drive southward from her location on Hardtack Avenue, towards the southeastern corner of the borough of Alderney. When you do, you'll go to the location of the penitentiary. It's at this point that Niko will spring the trap.

When the game instructs you via a prompt to initiate the kidnapping, turn the car around and go towards the new mark on your HUD, which is located at the north end of Alderney. Your ultimate goal is to bring her to a safe house in this location, off of Catskill Avenue, where some of Packie's friends are waiting to take the girl. This drive isn't easy initially, however, and the girl will put up a fight, grabbing the wheel and forcing Niko all over the road. When she does this, slow down (but don't stop for too long!) and regain control of the car. Eventually, she'll piss Niko off enough that Niko will knock her out, and from there, the drive becomes a hell of a lot easier. Once you arrive at your location, the mission comes to an end.

Please keep in mind that at some point, Packie will call you up asking you to head back to the safe house and take a picture with your phone of the captive girl (her father thinks she's staying with some guy and wasn't kidnapped). When you get these instructions, head back to the safe house (it'll appear on your HUD), walk on in, and take a picture, sending it to Packie thereafter. Once that's done, things having to do with this mission are truly finished.